The greenest cities should be, well, green. Leafy walls and roofs could help people turn down the air conditioning on hot days, saving huge amounts of energy. It could lower temperatures by up to 11 掳C, depending on the city.
Eleftheria Alexandri and Phil Jones at the Welsh School of Architecture at the University of Cardiff, UK, used computer models to compare the impact of 鈥済reening鈥� buildings in nine cities, including temperate London in the UK, humid Mumbai in India, and tropical Bras铆lia in Brazil.
Using temperature data from each city鈥檚 warmest month, they found that the air would be cooler around every building with green walls and roofs. And the hotter the climate, the greater the cooling effect (Building and Environment, ).
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If, for example, a group of buildings in Riyadh, Saudi Arabia, was clad in vegetation, the average temperature in the gap between the buildings would be 9 掳C cooler during the day and the top temperature in these spaces would fall by 11 掳C. In London and Montreal, peak temperatures would drop by 4 掳C.
Green surfaces lower local temperatures in two ways. Firstly, the green surfaces absorb less heat than typical building coverings, and so radiate less heat back into the immediate vicinity. Secondly, plants cool the air through the evaporation of water as they transpire.
Greening buildings is catching on. In Switzerland, for example, roofs covered in alpine plants that require little soil are increasingly popular, while wall-climbing plants adorn homes in many countries.
